Up to now, you had to have an invitation from a current user before you could sign on to use the innovative phone add-on service. Now, anyone in the U.S. can sign up.

Voice, which premiered in invite-only fashion a year ago, provides you with a single phone number that directs calls to your mobile, home or work phone, depending on the rules you set. Beyond that, it includes a basketful of features including transcribed voicemail sent as e-mail or text message, free conference calling, free outbound domestic calls and call-screening. The service is free, except for international calls, which cost a fraction of what the phone company charges per minute.

Google Voice used to be known as GrandCentral before Google bought the company in July 2007. Craig Walker, the former CEO of GrandCentral, now runs Google Voice, which he says has a million active users.

"That's a big number for a service that is invite-only," Walker said.

New users get a free new number, hopefully somewhere near their local area, and in anticipation of a rush, Voice has bought a bunch of numbers spread across the country (though Hawaii and Alaska remain "challenges").

"We are pretty optimistic there will be a high demand," Walker said.

Voice also has native apps for Android and Blackberry phones, while Palm Pre and iPhone users can use the HTML5 app (including to make outbound calls). Google made a full-featured app for the iPhone, which Apple controversially rejected on the grounds it would confuse users.

Those who commit to giving out a Google Voice number as their central point of contact aren't locked into Google and can port their number out to a phone company or to competing services such as Toktumi.

Users can't yet port their existing numbers into Google Voice, which would eliminate the need to have to "commit" to your Voice number by giving it out to friends and contacts, and getting new business cards made.

Voice could mature into a service that competes with Skype, which has become the world's largest telephone service by allowing free calls between Skype users and cheap calls from PCs and mobile devices to overseas phone numbers.

To that end, Google purchased a Skype competitor called Gizmo5 last November, which lets users make calls from their PCs to other PCs or to the traditional network.

Google is currently integrating this into Google Voice, though there is reportedly some internal friction over whether to scrap the downloadable software in favor of an HTML5 web app, even though the latter wouldn't have as good voice quality.

Currently, Google Voice calls use the minutes of a mobile phone plan, unlike Skype, which bypasses the voice channel by making calls using data plans.

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